Sterilization is a treatment that allows the destruction of vegetative forms and microbial spores and the prolonged conservation of a product stored at room temperature. Usually, heat treatments at higher temperatures than 100° C. are used. It is obvious that after a treatment with such conditions effects over the organoleptic properties can be seen, and in the case of food there can be important loses of nutritive value. On the other hand, many products cannot support these conditions and they physically destabilize. Food and other products sterilized by this system have a shelf life longer than six months (depending on its composition) if kept at room temperature. Sterilization process can be applied before or after packaging, requiring for each case different technologies as we will see later. Sterilization goes always along with food stabilization. In the case of solids, it is necessary to apply additives that protect colour and texture and strengthen flavour. In the case of liquid foods of colloidal nature, in order to avoid phase separation, mechanical treatments are employed such as the conventional homogenization and stabilizers are added (emulsifiers, feed thickeners, protectors for salt precipitation, etc.) depending on the food complexity.
In the case of sterilization of packed products, the heat treatment applies to the whole group of package and its content (food), and depending on the production requirements, a load system or a continuous one can be used.
When the food to be sterilized is a liquid the viscosity of which allows for it to be pumped, it is possible to use a sterilization system preceding packaging, associated to a subsequent aseptic packaging. In this case, the product circulates in a closed circuit in which there is a successive procedure of preheating, sterilization, cooling and aseptic packaging. Generally, sterilization is performed at high temperature: 135-150° C., which allows a very short time for processing: 4-15 sec. This treatment is usually known as Ultra High Temperature (UHT). UHT processes were implemented at industrial level on the 60's for liquid milk treatment, thus, obtaining products with characteristics more similar to the pasteurized milk than the ones obtained with conventional sterilizers that were used to sterilize bottled milk. From the 60's to the present day, others UHT processes have also been developed for other dairy products (concentrated milk, fresh cream, shakes, fermented products, ice creams, desserts . . . ) and for soups, sauces and purées, etc.
Compared with the sterilization of packed products, UHT process saves time, power, space and manpower. Nowadays, in the market there exist two UHT treatments: direct systems where the product is heated by direct contact with the heating medium (water steam), and indirect systems where heat is transmitted through a separation surface, in a heat exchanger.
In these sterilization processes and depending on the type of food, mainly on those that have an emulsion of the type oil in water (for example, milk, dipping sauces, shakes with dairy base or ice cream mix) it is necessary to introduce a homogenization process before or after the heat treatment. Homogenizers action reduces droplets size in the dispersed phase in order to stabilize the product in case there is a creaming phenomenon during its storage. Pressure homogenizers are built with a high pressure pump that works at 10-70 Mpa having a homogenization valve at the discharge side. When pumping liquid from the space between the valve and its support, the high pressure generated moves the liquid at high speed. At the valve end, the liquid movement speed drops abruptly and the extreme turbulence generated produces an intense shear rate. Other forces intervening in the process of reducing particle size are the collapsing of air bubbles (cavitation) and the impact forces created at the valves during the liquid trajectory. In some food, milk for example, there is sometimes an abnormal distribution of particles which produces additives. A second valve, similar to the first one and installed at the liquid trajectory, breaks these additives once more.
Heat treatment has, on the one side, beneficial effects over the food such as the microbial inactivation, however, in a parallel way; it generates undesired chemical and physicochemical changes, which can affect nutritional, organoleptic and/or technological properties depending on the applied treatment.
Flavour (aroma+flavour+consistency) is a very important parameter to consider as a quality aspect for the consumer of one of the most widely consumed sterilized food, such as milk, even more if it is consumed as a drink. Heat treatment has an important effect over milk flavour which can affect it in a higher or lower scale depending on the applied treatment intensity. An UHT sterilized milk (135-150° C., 2-20 s) is identified by a cooked aroma, mainly caused by the presence of H2S released after protein denaturalization, along with “caramel” aroma and another typical one associated to ketones formation. During a sterilization treatment in conventional bottle (105-120° C., 10-40 min) there is a strong cooked, ketone and caramel aroma, the later one caused by the formation of certain products from the Maillard reaction and caramelization products, which can even disguise cooked aroma. Other physical and biochemical phenomena these heat treatments can cause are product instability during its storage at room temperature due to protein precipitation, phases separation, creaming (separation of fat even though the product was previously homogenized), which makes it necessary for us to use certain additives such as emulsifiers, stabilizers or pH regulators in order to minimize or soften these effects derived from heat treatment. In less complex food, such as juices, there is a drastic loss of their vitamin content (vitamin C and other hydrosoluble vitamins), great alterations in their original flavour and aromas (volatile components loss), as well as changes in their colour.
Ultra High Pressure Homogenization (UHPH) Technology is based under the same principles as conventional homogenization with this one big difference that it can reach pressures higher than 200 Mpa, thanks to the valve design and the use of new materials. UHPH treatment can be associated to emerging physical techniques since its action results from combined forces of shear, turbulence, cavitation and impact caused by the application of dynamic high pressures. Nevertheless, this technology must not be mistaken with another technology that uses high pressures as well such as High Hydrostatic Pressure treatment (HHP). This technology, like UHPH, was developed as an alternative to the conventional heat treatments in the destruction of pathogenic and altering microorganisms, but the systems or work equipment as well as the mechanism of microbial inactivation which acts in this technology are totally different compared with UHPH as we will describe below. HHP equipment works with loads (discontinuous process) of product previously packed in flexible materials and closed guarantying their watertightness; this equipment is basically formed by a cylinder containing a pressure transmitting static fluid which is normally water (that is why it is called hydrostatic), a pressure generating system (low pressure pump and pressure intensifier). In this technology, packed food is introduced in the pressure cylinder filled with the pressure transmitting liquid (usually water) until selected pressure conditions are met, 400-1000 Mpa; (in industrial equipment of food applications up to 600 Mpa), and it is kept during the desired time. During this time, pressure is isostatically transmitted, which implies that the product is treated by homogenization, regardless its shape or size, and at the same time it prevents its deformation during the treatment. Then, after depressurising the cylinder, it is opened in order to extract the product from the machine.
As regards microbial inactivation mechanisms, HHP technology can inactivate microorganisms inducing changes in their morphology, biochemical reactions, genetic mechanisms or in their cell membrane. Normally, spores resist these treatments unless treatments combined with high temperatures are applied.
UHPH equipment developed so far are capable of processing fluids or pumpable food systems up to pressures of 400 MPa working in continuous processes. Up to now, different high pressure homogenization equipment has been employed in the chemical and pharmaceutical industries, specially food and biotechnology in order to emulsify, diffuse, mix and process their products.
In ultra-homogenizers, the homogenization valve is made with materials (such as ceramics) which are able to withstand pressures of up to 400 MPa (and its evolution to reach even higher pressures is probable) and temperatures over 100° C. Furthermore, the geometry of the valves is different from the classic APV-Gaulin valve found in conventional homogenizers.
This technology produces the disruption of dispersion particles including microorganisms. Particles can have a varied nature and are common in colloidal food such as milk, vegetables shakes, cloudy juices, etc. Among possible physical processes implied in microbial breakdown (main mechanism of microbial inactivation) during UHPH we can find: sudden pressure drop, impact forces, cut and torsion, turbulence and cavitation. The temperature increase of the product after passing through the valve contributes to microbial inactivation (including spores), since it is an additive effect to the physical forces developed at the homogenizer valve.
Even though we can consider UHPH technology as an alternative to the heat treatments, during UHPH process there is a marked increase of the product temperature due to: (1) the pressure increase occurring inside the intensifier and in the pipes located before the valve which generate a compression of the fluid and (2) the forces to which the fluid is subjected when passing through the high pressure valve and the conversion of kinetic energy into heat energy.
The pressure increase preceding the homogenization stage and the friction caused by the fluid high speed elevate product temperature approximately 2-2.5° C. every 10 MPa (a temperature increase of 20° C. to 50° C. in a homogenization cycle of 150 MPa). However, this heat effect, which applies in ultra-short periods (<0.5 s) can optionally be cancelled or minimized to the maximum by introducing a cooling equipment which, after the product pressure drop, controls temperature in a quick and efficient way. Likewise, that temperature increase caused by the homogenization cycle could be favoured and maximized if we expose the product to 40-90° C. temperatures, getting even sterilization temperatures (up to 150° C.), in flash way, after the first homogenization stage.
As regards the UHPH technology application in food fluids, it has been suggested that this treatment can cause pasteurization of several products such as milk, vegetable shakes, eggs, juices, etc. (Donsi, F., Ferrari, G., & Maresca, P. 2009. High-Pressure Homogenization for Food Sanitization. Chapter 19, pages 309-335. 2 In: Global Issues in Food Science and Technology. Ed. Barbosa-Cánovas, G. et al. Academic Press. Burlington, Mass., USA.). However, employed equipment and processes have shown to be insufficient to reach the sterilisation of studied products. For instance, Puig et al. (2008) studied the effect of UHPH treatment (200 MPa, inlet temperature 6-8° C.) over the microbiological and physicochemical characteristics of grape must, obtaining a residual microbiota in the product but with excellent sensory characteristics. Donsi et al (High-Pressure Homogenisation for Food Sanitisation. Proceedings of the 13th World Congress of Food Science and Technology ‘Food is Life’, Nantes, 17-21 Sep. 2006, 1851-1862, doi:10.1051/IUFoST:20060497) studied the effect of different UHPH cycles at 250 MPa in orange, apple and pineapple juices, evaluating microbial inactivation and quality loss of such treated products. UHPH was an effective treatment for obtaining pasteurized fruit juice, thus extending their shelf life and keeping their sensory characteristics for 28 days, refrigerating the product at 4° C.
Other researchers have suggested the addition of antimicrobial components to improve microbial inactivation produced by a UHPH treatment. In that way, Pathanibul et al. (2009. Inactivation of Escherichia coli and Listeria innocua in apple and carrot juices using high pressure homogenization and nisin. International Journal of Food Microbiology 129, 316-320.) studied the addition of nisin (0-10 Ul/ml) in apple and carrot juice inoculate with Escherichia coli or Listeria innocua (˜7 log ufc/ml) and treated by UHPH (0-350 MPa), they observed important microbial reductions (˜5 log ufc/ml) but their complete elimination was not reached.
The problem of ultra-homogenizers is that they do not guarantee by themselves the sterilization and subsequent packaging of food in aseptic conditions. That is to say, it is necessary to combine a series of equipment in a “System” which allows the sterilization (including the destruction of resistance spores), the stabilization without additives or with a higher control of their concentration and the packaging in aseptic conditions.